FIX: No method error in WebhooksController#sendgrid (#31495)

When an email is sent by sendgrid to an email address with an invalid
host, the webhook payload does not contain the "status" field:

```
[
  {
    "bounce_classification": "Unclassified",
    "email": "noemail@this.does.not.exist.tld",
    "event": "bounce",
    "reason": "unable to get mx info: failed to get IPs from PTR record: lookup <nil>: unrecognized address",
    "sg_event_id": "Ym91bmNlLTQtNTA0ODUxOTUtZXVvMmlLeGRTYXlQRjRZRTQtLUk3QS0w",
    "sg_message_id": "euo2iKxdSayPF4YE4--I7A.recvd-5f54b5d587-pczjm-1-67BADEEA-6.0",
    "smtp-id": "<870b3a2a-160c-4fc8-bc9a-bd0d5b943b81@forum.umbraco.com>",
    "timestamp": 1740300320,
    "tls": 0,
    "type": "blocked"
  }
]
```

When the `status` field is missing, it results in a `NoMethodError
(undefined method `[]' for nil:NilClass)`
error in the controller method. In this commit, we will specifically
handle the webhook event from sendgrid when the email address's domain
is invalid.

Co-Authored-By: @nul800sebastiaan

app/controllers/webhooks_controller.rb

@@ -25,7 +25,12 @@ class WebhooksController < ActionController::Base
       message_id = Email::MessageIdService.message_id_clean((event["smtp-id"] || ""))
       to_address = event["email"]
       error_code = event["status"]
+
       if event["event"] == "bounce"
+        # Sendgrid does not provide status field for emails that can't be delivered due to the recipient's server not existing
+        # so we set the error code to 5.1.2 which translates to permanent failure bad destination system address.
+        error_code = "5.1.2" if !error_code && event["type"] == "blocked"
+
         if error_code[Email::SMTP_STATUS_TRANSIENT_FAILURE]
           process_bounce(message_id, to_address, SiteSetting.soft_bounce_score, error_code)
         else

spec/requests/webhooks_controller_spec.rb

@@ -3,8 +3,8 @@
 RSpec.describe WebhooksController do
   before { Discourse.redis.flushdb }
 
-  let(:email) { "em@il.com" }
+  fab!(:email) { "em@il.com" }
-  let(:message_id) { "12345@il.com" }
+  fab!(:message_id) { "12345@il.com" }
 
   describe "#mailgun" do
     let(:token) { "705a8ccd2ce932be8e98c221fe701c1b4a0afcb8bbd57726de" }
@@ -82,10 +82,10 @@ RSpec.describe WebhooksController do
   end
 
   describe "#sendgrid" do
-    it "works" do
+    fab!(:user) { Fabricate(:user, email:) }
-      user = Fabricate(:user, email: email)
+    fab!(:email_log) { Fabricate(:email_log, user:, message_id: message_id, to_address: email) }
-      email_log = Fabricate(:email_log, user: user, message_id: message_id, to_address: email)
 
+    it "works" do
       post "/webhooks/sendgrid.json",
            params: {
              "_json" => [
@@ -106,6 +106,26 @@ RSpec.describe WebhooksController do
       expect(email_log.user.user_stat.bounce_score).to eq(SiteSetting.hard_bounce_score)
     end
 
+    it "sets the bounce error code to 5.1.2 when payload's `event` is `bounce`, `type` is `blocked` and `status` is blank" do
+      post "/webhooks/sendgrid.json",
+           params: {
+             "_json" => [
+               {
+                 "email" => email,
+                 "smtp-id" => "<12345@il.com>",
+                 "event" => "bounce",
+                 "type" => "blocked",
+               },
+             ],
+           }
+
+      expect(response.status).to eq(200)
+
+      email_log.reload
+      expect(email_log.bounced).to eq(true)
+      expect(email_log.bounce_error_code).to eq("5.1.2")
+    end
+
     it "verifies signatures" do
       SiteSetting.sendgrid_verification_key =
         "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE83T4O/n84iotIvIW4mdBgQ/7dAfSmpqIM8kF9mN1flpVKS3GRqe62gw+2fNNRaINXvVpiglSI8eNEc6wEA3F+g=="